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<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
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<body>
  <script>
    // let age = 0;
    let age;
    console.log(age);// undefined
    age = 25;
    console.log(age);//25
    age = 20;
    console.log(age);//20
  </script>
</body>

</html>